Various methods have been employed to implement devices for sensing infrared radiation. Among infrared sensors, microbolometer infrared sensors are notable, because of the method used to sense infrared rays; by measuring an electric signal that changes according to temperature differences caused by absorbed heat from infrared rays.
Typically, a non-cooled infrared sensor such as a micro-bolometer includes a light receiving portion and a detecting portion. The light receiving portion converts energy of photons into heat, and the detecting portion detects electric signals, such as resistance, capacitance, and pyroelectric effect, which change according to the temperature caused by the heat converted by the light receiving portion.
The conventional infrared sensors, especially, a resistive sensor includes a resistive detecting element which may be vanadium oxide (VOx), poly-silicon, amorphous-silicon, thermistor (MnNiCO)3O4 or a diode. VOx has an advantageously good resistance change rate to temperature change, whereas it has a fatal disadvantage that the flicker noise is excessively large. Thus, the conventional resistive infrared sensor using such material as the resistive detecting element, has system performance that drastically deteriorates (especially at a low frequency where the influence of flicker noise substantially increases). To solve the problem such as the flicker noise, titanium with a very small flicker noise has been used for the resistive detecting element. However, since material such as titanium has a too small resistance change rate to temperature change, it is not suitable for the detecting element of the infrared sensor.
Furthermore, according to the conventional art, sometimes a mechanical chopper is externally provided to the infrared sensor to improve the sensor's performance in sensing infrared radiation. This is used principally for ferroelectric devices and sometimes (but not necessarily) for microbolometers. The mechanical chopper periodically cuts off photons to maintain a detecting element below a reference temperature. The mechanical chopper, when used, greatly increases the complexity of an infrared detecting system. That is, the mechanical chopper complicates the assembly processes of the infrared sensing system, increases the manufacturing cost, and causes various limitations in operation.
Moreover, the semiconductor device includes a detecting element that outputs electric current, and the output electric current responding to light has a significantly weak intensity. Since the magnitude of a common mode current which is output in dark conditions is significantly large and the common mode current itself changes according to temperature, it is difficult to accurately measure a sensing current output from a sensor.
Furthermore, according to the conventional art, all non-cooled thermal imaging detectors suffer from image lag, where in image lag is the remaining parts of one frame of a video still being present in the current frame of video. This effect manifests itself as a smearing and ghosting.